1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly, to a cholesteric liquid crystal (CLC) display device and a method for manufacturing the same in which a circular polarizer is formed in an interior of a liquid crystal cell resulting in a decrease of a thickness of the cholesteric liquid crystal (CLC) display device and reduced light leakage.
2. Discussion of the Related Art
As the information age has rapidly evolved, the need for flat panel displays, which have advantages such as thinness, light weight and lower power consumption, has increased. Liquid crystal display (LCD) devices have been applied to notebook computers, desktop monitors, etc., because of their superiority in resolution, display of color images and displaying quality.
Generally, the liquid crystal display device has upper and lower substrates, which are spaced apart and face each other. Each of the substrates includes an electrode, and the electrodes of each substrate face each other. Liquid crystal is interposed between the upper substrate and the lower substrate. Voltage is applied to the liquid crystal through the electrodes of each substrate, and thus alignment of the liquid crystal molecules is changed according to the applied voltage to display images. Because the liquid crystal display (LCD) device has polarizers that transmit only light of a particular direction on an outer side of both substrates, a gray scale of transmitted light can be controlled. Because the liquid crystal display device cannot emit light by itself, it needs an additional light source to display images. Accordingly, the liquid crystal display device has a back light as a light source behind a liquid crystal (LC) panel. The amount of light transmitted from the back light through the LC panel is controlled according the alignment of the liquid crystal molecules in the LC panel to display images. The electrodes of each substrate are formed of transparent conductive material, and the substrates must be transparent. However, the polarizer of this liquid crystal display (LCD) device transmits light of certain direction but absorbs light of other direction. The absorbed light is lost as heat, etc., and thus a luminance loss of over 50% is induced considering a reflection on a surface of the polarizer. Accordingly, the liquid crystal display (LCD) device having a reflective circular polarizer has been suggested in order to increase the luminance by reducing the luminance loss. The reflective circular polarizer transmits a certain circularly polarized component of an incident light and reflects the other circularly polarized component of the incident light. The reflected circularly polarized component is reflected again while it passes through many optical elements under the circular polarizer, and a direction of the reflected circularly polarized component is changed so that it can be transmitted through the circular polarizer. That is, because a whole amount of the incident light can theoretically be transmitted through the circular polarizer, the loss of the incident light can be greatly reduced compared with the liquid crystal display (LCD) device having only an absorptive linear polarizer. Besides, the cholesteric liquid crystal (CLC) display device, which has a cholesteric liquid crystal (CLC) color filter and the circular polarizer, has been researched and developed. Because the cholesteric liquid crystal (CLC) has a selective reflection property, the luminance can be improved compared with the liquid crystal display (LCD) device having an absorptive color filter. A structure of a related art cholesteric liquid crystal (CLC) display device will be described hereinafter with reference to attached figures.
FIG. 1 is a cross-sectional view of a related art cholesteric liquid crystal (CLC) display device. The cholesteric liquid crystal (CLC) display device has a transparent first substrate 11 and a transparent second substrate 14 that are spaced apart and facing each other. A cholesteric liquid crystal (CLC) color filter 12 is formed on the first substrate 11. A first electrode 13 is formed on the cholesteric liquid crystal (CLC) color filter 12 and a second electrode 15 is formed beneath the second substrate 14. A liquid crystal layer 16 is disposed between the first and second electrodes 13 and 15. The first and second substrates 11 and 14, other layers therebetween and the liquid crystal layer 16 form a liquid crystal cell. A circular polarizer 21, which is formed of cholesteric liquid crystal (CLC), is adhered to the first substrate 11. The circular polarizer 21 has a third substrate 21a, an adhesive layer 21b and a cholesteric liquid crystal (CLC) layer 21c. The cholesteric liquid crystal (CLC) layer 21c is adhered to the third substrate 21a by the adhesive layer 21b. A diffusive film 31 is formed on the second substrate 14 in order to diffuse light. A retardation layer 32 and a linear polarizer 33 are sequentially formed on the diffusive film 31. A back light 40 is disposed under the circular polarizer 21. The back light 40 has a light guide panel 41 and a lamp 42 that is at one side of the light guide panel 41. The back light 40 desirably has a high collimating pattern. The light irradiated from the back light 40 is circularly polarized when it passes through the circular polarizer 21, and then only partially circularly polarized light is transmitted by selective reflection and transmission when it passes through the cholesteric liquid crystal (CLC) color filter 12. Because partially reflected light is reflected again by many layers under the cholesteric liquid crystal (CLC) color filter 12 and then transmits through the liquid crystal cell, the light usage efficiency is improved. A color shift problem of the cholesteric liquid crystal (CLC) display device can be overcome if collimated light is used for the incident light. Because the collimated incident light can be diffused by disposing the diffusive film 31 on the liquid crystal cell, the luminance of the liquid crystal display (LCD) device can be increased.
As described above, the circular polarizer 21 is formed by adhering the cholesteric liquid crystal (CLC) layer 21c on the separate third substrate 21a using adhesive layer 21b and the circular polarizer 21 is adhered to the first substrate 11 using an another adhesive layer 22. Thickness of the third substrate 21a is between about 40 μm and 80 μm, the cholesteric liquid crystal (CLC) layer 21c is about 10 μm and the adhesive layers 21b and 22 is between about 10 μm and 20 μm. Accordingly, the thickness of the liquid crystal display (LCD) device becomes thick. Furthermore, light leakage occurs in the third substrate 21a and the adhesive layer 21b and 22, resulting in a decrease in luminance.